The present invention generally concerns a low frequency quartz oscillator device.
In the following description, xe2x80x9cquartz oscillator devicexe2x80x9d means an oscillator device including a quartz resonator associated with oscillating means, or an electronic circuit for maintaining the vibrations of the resonator.
Those skilled in the art know various types of quartz oscillator devices. Those skilled in the art know, in particular, oscillator devices using a quartz resonator arranged to vibrate according to a flexural vibration mode. Such resonators typically have parabolic type thermal characteristics and are relatively sensitive to temperature variations.
In order to overcome this drawback, those skilled in the art know that the sensitivity of such oscillators to changes in temperature can be compensated for or at least reduced by additional means, or a temperature compensation circuit. In addition to the fact that these additional means increase the cost of such oscillators, it is to be noted that the power consumption of these devices is also substantially increased. Moreover, these additional temperature compensation means typically operate by adjusting the load capacitor or capacitors of the resonator or by adding or eliminating oscillation pulses. This has the effect of greatly degrading the purity of the frequency spectrum of the oscillation signal. Indeed, the appearance of a large number of spectrum lines of quite significant width is observed, the position of these lines also varying with the temperature.
These oscillator devices cannot be used in cases where it is necessary to have an oscillation signal having not only a temperature stable frequency but also a frequency spectrum including only a low number of spectrum lines. A signal having these properties is for example necessary in telecommunications to allow a synchronisation operation.
Those skilled in the art also know oscillators including a so-called AT cut quartz resonator with cubic type thermal characteristics and whose frequency is very stable as a function of the temperature. However, by nature, this frequency is quite high, of the order of several MHz. Consequently, in order to use such an oscillator device to supply a low frequency oscillation signal, the oscillator needs to be fitted with a frequency divider circuit, which complicates and increases the cost of the device. Furthermore, the electric power consumed by the frequency divider circuit is relatively significant because of the high frequency of the signal which it receives at its input, which proves to be a serious drawback when the power has to be supplied by an autonomous power source of small dimensions such as a wristwatch battery.
A general object of the present invention is thus to propose a quartz oscillator device which overcomes the aforementioned drawbacks, i.e. an oscillator device which generates an oscillation signal having good thermal characteristics and good spectral purity and which preferably consumes little power.
The present invention therefore concerns a quartz oscillator the features of which are listed in claim 1.
The present invention thus proposes, firstly, using a torsional type quartz resonator, i.e. a resonator arranged to vibrate according to a torsional vibration mode. According to the present invention, this resonator is, in particular, a resonator of the type described in the article by Messrs. Roger Bourquin and Philippe Truchot entitled xe2x80x9cBarreaux de quartz vibrant en mode de torsion, Application aux capteursxe2x80x9d, 6th European Chronometry Congress, Bienne, 17-18 October 1996, which is incorporated herein by reference.
FIG. 1 annexed hereto shows a non limiting example of such a torsional type resonator, globally indicated by the numerical reference 1. This resonator 1 has the shape of a tuning fork obtained by chemical etching or mechanical machining of a quartz plate along a determined cutting angle so that the branches of the resonator are oriented in the crystallographic plane YZ of the quartz crystal at a determined angle xcex8 as is shown clearly in FIG. 1.
This type of resonator has the advantage of better thermal characteristics compared to conventional flexural vibrating resonators. In particular, the thermal characteristics of this torsional vibrating resonator are determined by the cutting angle and by the thickness over width ratio (t/w) of the arm. By way of example, a torsional xcex8 vibrating tuning fork resonator made in accordance with the teaching of the aforementioned article allows better thermal stability to be obtained, of the order of a factor of 3, compared to a conventional flexural vibrating tuning fork resonator.
It will be noted that there is also known another type of resonator arranged to vibrate according to a torsional mode. This other torsional vibrating resonator is known by the name of a TT-cut torsional quartz resonator. Such a resonator is for example described in the article by Messrs. Hirofumi Kawashima and Mitsuhiro Nakazato entitled xe2x80x9cTT-Cut Torsional Quartz Crystal Resonatorxe2x80x9d, 45th Annual Symposium on Frequency Control xe2x80x9cASFC), IEEE, 1991.
Unlike the torsional vibrating resonator of the preceding article, which is used within the scope of the present invention, this resonator is characterised by two cutting angles and requires a more complex electrode structure. In terms of manufacturing simplicity and cost, the torsional vibrating resonator described in the aforementioned article by Messrs. Bourquin and Truchot thus constitutes a more advantageous solution.
It will be noted however that a drawback of the torsional vibrating resonator described in the first aforementioned article resides in the fact that, in addition to the desired fundamental torsional vibrating mode, it has undesired flexural vibrating modes. As will be seen in detail hereinafter, this type of resonator has, in particular, a fundamental flexural vibrating mode at a substantially lower frequency than the frequency of the desired torsional vibrating mode. Consequently, if a resonator of this type is associated with a conventional electronic maintenance circuit, the assembly will in practice oscillate according to this fundamental flexural mode and not according to the desired fundamental torsional mode.
The present invention thus also proposes to answer this drawback of the aforementioned torsional vibrating resonator, namely to provide an electronic maintenance circuit for the resonator vibrations assuring that the resonator actually vibrates according to the desired fundamental torsional vibrating mode.
According to a particularly advantageous embodiment of the resonator, the geometry of the resonator is selected so that the desired fundamental torsional vibrating mode is located substantially close to 393,216 kHz, i.e. 12 times the frequency of 32,768 kHz which is the typically frequency of a quartz resonator intended for horological applications.
These objects, features and advantages of the present invention, in addition to others, will appear more clearly upon reading the following detailed description, made with reference to the annexed drawings, given by way of non limiting example and in which: